1. Field of the Invention
The present invention relates to a reflection prevention type of optical connector.
2. Description of the Related Art
In recent years, an optical fiber is primarily used as a transmission line in the field of communications in response to an increase in transmission capacity. The connection of optical fibers is conventionally made by a splicing method such that the opposed end surfaces of the optical fibers are permanently connected together by splicing, or by a method using a detachable optical connector to connect the optical fibers together. In the latter method using the optical connector, it is required to reduce the misalignment of the optical fibers to less than about 1/10 of the core diameter of each fiber and also required to make good close contact of the opposed end surfaces of the optical fibers. Further, the demands on the optical connector are simple configuration, easy operation, and small connection loss.
Usually, the connection of optical fibers by the use of such an optical connector is carried out in the following manner. First, an optical connector is assembled by inserting and fixing in a centered ferrule a coupling end portion of an optical fiber to be connected, and then mounting the ferrule in a connector housing. Another optical connector is similarly assembled. Thereafter, the two optical connectors are inserted into an adapter from both ends thereof until the ferrules inserted in the connectors come to abutment against each other, thereby establishing the optical coupling between optical fibers.
In such a conventional optical connector, one ferrule is usually mounted in a housing, and the coupling end surface of the ferrule in the connector is generally adapted in polished condition to the coupling end surface of the ferrule in the other mating connector to be connected. Usually, the polished condition of the coupling end surface is spherical, and also in the following description, such a spherically polished condition of the coupling end surface will therefore be employed.
Conventional optical connectors with such structure pose two potential problems in operation. One problem is the radiation from the end surface of the connector when the connector is disconnected from the adaptor, which may cause damages of human eyes, depending on the radiated power, radiation angles, and accessibility to the connector end surface. Another is the reflection occurring at the end surface when the connector is open. Because of the refraction index difference, silica-air interface reflects about 5% of the transmitted light back to transmitter.
In SDH(Synchronous Digital Hierarchy) systems, those problems do not become serious, since in such systems optical output power is relatively low, and since they use the optical fiber for only one direction.
However, those two problems can become real problems in new systems; optical CATV systems or gigabit transmission systems which use optical amplifier, or bidirectional transmission system over single fiber.
First of the two problems will now be described with reference to FIG. 8.
A light signal transmitted from an optical CATV transmitting device 2 is branched by an optical star coupler 3 and is then received by optical receiver 4 on the subscribers side. However, if the optical branching device 3 has an open connector 5, or an optical connector 6 on any subscriber side is open, the light signal transmitted is reflected on the end surface of the connector 5 or 6 as shown by dotted arrows in FIG. 8. To prevent such reflection, unused connectors of the star coupler and/or the other end of the optical transmission line must be terminated properly.
In an optical CATV system using analog modulation, reflection at the open end surface degrades its transmission quality in that it increases distortion and lowers C/N ratio of the signal. In addition, the existence of reflection is likely to activate laser shutdown mechanism which detects open connector by reflection and shuts down the transmitter for laser hazard safety.
Activation of laser shutdown is good in itself in terms of operators' safety, but it at the same time means that all subscribers suffer the loss of signals.